In the European Union, the net neutrality regulation makes sure that user "can use the equipment of their choices" (Tim Wu (2003). "Network Neutrality, Broadband Discrimination" (PDF). Journal on Telecom and High Tech Law. page 169: "rule barring the Bells from preventing attachment of equipment unless harm to the network could be shown") to access the network.

So while there is some IMEI blocking in practice, as for instance the IMEI 00000000[...][1] is blocked in many networks, it's not legal to block devices based on their IMEI if that IMEI is valid and that the device is not stolen, etc.

For instance Operators aren't supposed to block older devices, devices from less known brands, devices from a specific brand, devices not tested or approved by the operator, etc.

We don't have any report of less common mobile phones like the Openmoko, or the GTA04 being blocked.

References:

1 This has been observed with OsmocomBB, by forgetting to add the IMEI inside the configuration file and trying to register, which didn't work. Once the proper IMEI (which is written on the back of the phone below the battery) was added to the configuration file, registering to the network worked.